1. Field of the Invention
The present invention pertains to electromagnetic heads for the reading/writing of information on a relatively moving magnetic recording medium, and more particularly, it pertains to the method of manufacturing such magnetic heads by controlling the throat height so as to achieve the desired magnetic characteristics through the manufacturing processes and in the finished product.
2. Description of the Prior Art
Magnetic recording heads, for reading from or writing on a moving medium, such as a disk or tape, have received a considerable amount of attention recently due to the ever increasing requirements for storing a larger amount of information within a given surface area of the medium and the consequent dimensional requirements imposed on the head to enable it to magnetically transfer information at the required rates. Such magnetic heads generally are comprised of a core element forming a continuous loop magnetic flux path having a gap therein in the submicron range, an electrically conductive coil wrapped about the core element to create flux movement in the core or to be energized by such flux movement in the core, and a mounting element (slider) mounting the core and the winding thereof in a position so that the gap faces and rides upon an air bearing surface over the moving medium during reading/writing of information.
The magnetic performance of a magnetic recording head, as determined by the magnetic flux behavior within the core element magnetic loop path, is directly related to the nature and dimensions of the materials selected to be used in the magnetic loop and particularly the dimensions of the gap therein. The throat height of the gap, i.e., the dimension of the gap perpendicular to the surface of the recording medium, primarily functions to optimize the write field as seen by the medium so as to cause the magnetic transitions written into the medium by the write field from the core to be accurately and consistently formed and placed. It has generally been assumed heretofore that in the manufacture of magnetic heads the various parameters involved, e.g., the magnetic properties of the recording medium and the dimensions and spacing of the various elements of the head, were constant and that thus there would be one best mechanical dimension for the throat height which would provide the optimum performance for the particular magnetic head being designed. It may be demonstrated, however, that the various magnetic and physical parameters involved for the various materials employed to fabricate the core at each of the many core manufacturing processing steps are affected by normal variations in material and the processing thereof. Thus, as a result of such many dimensional and magnetic variations which may exist during the manufacturing process, it can be shown that a unique value for the throat height dimension may be necessary for each particular magnetic head. This is particularly true in current manufacturing processes where very thin film heads are used with exceedingly fine spacing and dimensions of the head elements.
In the past, magnetic cores were larger, the tracks were wider, gaps were wider, flux changes per second per inch of medium were lower, gap to recording medium distances were greater, and medium thicknesses were greater; thus, the required throat heights were larger and the variations in the dimensional and magnetic parameters were less critical to the performance of the core in the magnetic head. Under these conditions, a nominal value of throat height could be chosen, and this was deemed sufficient for use in all cores fabricated for use in a particular recording system despite variations in the parameters due to manufacturing tolerances. As magnetic recording has evolved to the present level of packing densities and transfer rates, the control and measurement of the dimensional and magnetic material parameters for the required cores has become increasingly demanding. Throat heights are still established by the machining methods of grinding and/or lapping, but the machines employed have become increasingly sophisticated and the applied processes increasingly difficult to monitor and control. The requirements of dimensional measurement have passed the limits of resolution for the conventional microscopes that are considered both affordable and useable in the production environment.
The early method of controlling the throat height in a magnetic head was to grind and lap to machine and tooling tolerances and to measure with low power microscopes. This was acceptable when the values for the throat heights were larger than several milliinches. As throat height dimensions entered the micron range and even fractions of a micron, machining, tooling, and process variations forced the evolution of an iterative technique consisting of lap, measure, adjust, lap, measure, adjust, etc., until the specified throat height dimension was obtained. The measurement method was still an optical technique, but the optical measuring equipment was now of a special design. In general, no attempt was made to look at the magnetic performance of a particular magnetic head in order to adjust the throat height therefor.
With the advent of thin film heads, the throat height dimensional values were further reduced. Optical reference patterns were included as a part of the masks used in the production of the thin film elements. These optical guides, which are located on the wafers adjacent to the thin film elements, determine the throat height dimension as a function of the lapping process by the iterative method of lap/look/measure. However, because of the continued requirement for smaller throat height dimensional values, the resolution limits of affordable optical measuring equipment to accurately view the guides limited the effectiveness of these methods.
The preferred method today of controlling the throat lap height in the magnetic head manufacturing industry is through the use of a resistive element placed alongside the head which element can be electrically monitored as its length is changed during the grinding or lapping process as shown, for example, in U.S. Pat. No. 4,675,986 to Yen. The resistive elements are placed on the wafer adjacent to the magnetic elements as were the optical lapping guide patterns previously described. The electrical values of the resistive elements are a function of the materials used, the thickness of the resistive layer, and any variations in these parameters caused by the wafer making and machining processes. Because of this last factor, the resistive lapping guide method is best done by using a ratio approach or rate of change of resistive value as opposed to lapping to a specific value of resistance as the throat height indicator. Even so, the resistive guide can only provide a predetermined desired throat height and cannot take into account the processing and material variations in the head materials normally encountered.
Some work has been done toward using the inductance of the winding in the core element as the throat height lapping control method. However, there are several problems associated with this approach. Since most of the inductance in the core winding comes from a magnetic material directly within the winding and since the gap and throat height area are not within the winding, the inductive changes are not directly related to changes in the throat configuration. As the track widths become narrower, and hence the thickness of the core elements correspondingly decrease, the inductive change due to throat height dimensional value change becomes less definitive.
In a paper coauthored by the present inventor entitled "Pulse Transfer Analysis of Ferrites For Recording Head Applications", Proceedings of the Sixth International Conference on Ferrites, Tokyo, Japan, 1992, a method was proposed that allows monitoring of the appropriate magnetic properties of various ferrites used in magnetic head applications. This method involved the use of a pair of separate windings placed about a toroid of ferrite material. An electrical test pulse or series of pulses are applied to one of the windings and the response pulse or pulses induced in the other winding is detected and separately analyzed under varying processing factors such as lapping, polishing, annealing and film deposition, and varying combinations of the foregoing.